1. Field of Invention
The present invention relates to a bearing arrangement for use in a gear pump having an axially directed bearing surface including a region which acts to separate inlet and outlet ports of the pump. In particular, the invention relates to a bearing arrangement for use in a gear pump of the type used in an aircraft engine.
2. Description of Related Art
A conventional gear pump having a pair of intermeshed, externally toothed gear elements, is described in GB 2012876 A. Bearing blocks are used in pairs to provide a bearing surface for the teeth and drive shafts of the gears. Each gear tooth has a xe2x80x9cworking flankxe2x80x9d which engages the teeth of the other gear and a trailing, xe2x80x9cnon-workingxe2x80x9d flank, a small clearance referred to as the xe2x80x9cbacklash gapxe2x80x9d being defined between the non-working flank and the other gear. During operation, there are periods for which two working flanks of one gear are in engagement with working flanks on the other gear, causing a small quantity of fluid to be trapped in a volume, referred to as the inter-tooth volume, between the inter-engaged working flanks. The inter-tooth volume, defined between the two pairs of working flanks which are in contact, includes the backlash gap.
As the gears rotate, the inter-tooth volume first decreases and subsequently increases. During this volume change, the proportions of the inter-tooth volume on respective sides of the backlash gap also alter. The volume changes within the inter-tooth volume result in transient pressure changes in that volume. Such pressure changes result in air coming out of solution and the creation of vapour bubbles from the pumped fluid. A subsequent increase in pressure will result in collapse of the vapour bubbles. It is a particular problem in aircraft, when the pump is being used at high altitudes, that the collapse of the vapour bubbles causes cavitation erosion of the portions of the bearing surface which act to separate the pump inlet and outlet. The erosion of the end face of the bearing surface increases the leakage path between the outlet and inlet sides of the pump, therefore reducing pump efficiency. The presence of the leakage path reduces the efficiency of the pump and requires premature replacement of the bearing arrangement.
It has been proposed to overcome the problem of erosion of the bearing surface in this way by forming the bearing blocks, or the end faces of the bearing blocks, from a hard material. However, the forming of the bearing from a hard material can detract from the primary function of providing a bearing surface for the gears.
U.S. Pat. No. 4,311,445 describes a bearing arrangement for a gear pump of the type comprising a single bearing block having a xe2x80x98figure of eightxe2x80x99 configuration. A hard insert extends over the vulnerable region of the bearing surface in the form of a pin set in a central region of the block. However, as the arrangement includes only a single bearing block, it is not possible to profile the central region to further reduce the cavitation problem described previously. Furthermore, it can be desirable to locate drillings near the region of engagement of the gear teeth to provide a path for lubrication fluid to flow to the paths and it is not possible to do this in a figure of eight bearing block. A further disadvantage of this arrangement is that, if the hard pin is not perfectly flush with the surrounding bearing surface or comes loose through vibration and contacts the gear teeth, a catastrophic failure of the pump may occur. The use of the gear pump as a fuel pump for an aircraft engine cannot therefore be contemplated.
It has been previously proposed to mitigate the effects of cavitation damage in a twin bearing block arrangement by locating a hard steel insert in each of the two blocks. Each insert is located in an end portion of respective cylindrical lead-bronze bearing blocks and is secured therein by means of a locking pin which extends through a drilling in the mating surfaces of the blocks. Whilst the provision of the locking pin prevents the insert becoming loose, in order to avoid the possibility of the insert contacting the rotating gears, it is necessary for the insert to be located fractionally below the bearing surface. The small clearance between the surface of the insert and the bearing surface for the gear teeth and gear shafts provides a leakage path for fuel which reduces the efficiency of the pump. Furthermore, the bearing arrangement is expensive to manufacture as a large portion of the steel liner must be removed by machining following assembly to profile the outer surface of the block.
It is an object of the present invention to provide a bearing arrangement for a gear pump suitable for use in an aircraft engine in which the aforementioned problems are reduced or avoided.
According to a first aspect of the present invention, a bearing arrangement for a gear pump comprises first and second separate bearing blocks each having a respective block housing, the first and second bearing blocks together defining an axially directed bearing surface including a first region formed from an erosion resistant material which acts to separate inlet and outlet ports of the pump and a second region formed from an anti-friction material, wherein the first region is defined by first and second insert members cast within respective ones of the block housings, the first and second bearing blocks being arranged such that respective surfaces of the first and second insert members mate with one another, the first and second insert members being arranged such that the first region of the bearing surface is flush with the second region.
The casting of the insert members within the block housings and the subsequent machining of the cast insert member and block housing arrangement ensures the bridge region is flush with the remainder of the bearing surface, thereby ensuring substantially no leakage path exists between the outlet and inlet sides of the pump which would otherwise reduce pump efficiency. Additionally, as the insert members are cast within the block housings, there is no need for a separate hard liner and separate locking pins to secure the insert members in place. The cost of manufacturing the bearing arrangement is therefore reduced considerably.
In a preferred embodiment of the invention, the erosion resistant material is stainless steel.
Preferably, the block housings are cast from a lead-bronze material such as, for example, Tokat 30.
Each of the bearing blocks is preferably provided with an inlet relief in communication with the inlet of the pump and an outlet relief in communication with the outlet of the pump, the bridge region of the bearing surface being defined between the inlet and outlet reliefs.
One or more of the relieved regions of the bearing surface is preferably provided with a drilling or passage which communicates with a through bore provided in the respective bearing block to provide a supply of lubricating fluid within said bore.
According to a second aspect of the present invention, a bearing arrangement for a gear pump comprises first and second separate bearing blocks each having a respective block housing, the first and second bearing blocks together defining an axially directed bearing surface formed from a reduced friction material, except for a bridge region formed from an erosion resistant material which acts to separate inlet and outlet ports of the pump, wherein said bridge region is defined by first and second insert members cast within respective ones of the block housings and machined to be flush with the remainder of the bearing surface.
According to another aspect of the present invention, a method of assembling a bearing arrangement for a gear pump comprises the steps of;
providing an insert member, including a projection, formed from an erosion resistant material,
casting a reduced friction material over the insert member, to provide a cast block housing, such that the projection protrudes from an end face of the cast block housing,
machining the end face of the cast block housing from which the projection protrudes such that the projection is detached from the remainder of the insert member, thereby to define an axially directed bearing surface comprising a first region defined by the insert member and a second region defined by the cast block housing, wherein the first region is substantially flush with the second region.
Preferably, the method includes the further step of coating an outer surface of the cast block housing with a coating material.
The method may further comprise the step of machining the coated outer surface of the bearing block to achieve the desired profile.
It will be appreciated that the aforementioned preferred features of the first aspect of the invention apply equally to the other aspects of the invention also.